Reduction of noise



March 28, 1 c. M. BURRILL 2 REDUCTION OF NVOISEV 30 H 2 Shegts-Sheet 1 Zinnentor Charles M. Barri Ll Gttomeg March 28, 1939i. c. M. B URZRILL 2,151,740 I REDUCTION OF NOISE Filed April 50, 1936 2 Sheets- Sheet 2 SLOW CIRCUIT Zhwentor Charla-S M. Burrill attorneg UNITED STATES PATENT OFFICE REDUCTION OF NOISE Charles M. Burrill, Haddonfield, N. J., assignor to Radio Corporation of America, a corporation 'of Delaware Application April 30, 1936, Serial No. 77,117

8 Claims.

This invention relates to the reduction of noise such as that produced in radio receivers and like apparatus as the result of electrical impulses extraneous to the desired signal. It has for its principal object the provision of an improved apparatus and method of operation whereby a noise suppressing impulse supplied through a relatively high velocity control channel is utilized to make the major portion of the receiver incapable of shock excitation by the noise. In this way the noise suppression is applied before the duration of the noise impulse has been-increased to any considerable extent by the selectivity of the receiver. .Among the further objects is the provision of a noise reduction system which is especially applicable Where the effect of localized interference, such as that of an internal combustion engine ignition system, is to be avoided, and which is relatively simple and inexpensive. 2 In noise reduction systems using rapid acting control methods, it is desirable to introduce the control at a point as near the receiver input as practical. In other words, the selectivity preceding the control point should be as small as possible, and the selectivity following the control point should be as great as possible for the reason that selectivity preceding the control point increases the duration of an initially very short noise impulse, and necessitates the application of the control for a correspondingly longer time. On the other hand, selectivity following the control point aids in fiy-wheel-like manner in filling in the breaks in the continuity of reception produced by the operation of the noise reduction 35 control, and so minimizes their deleterious effect.

The above principle of apportionment of selectivity applies to the reduction of impulsive in terference substantially greater in amplitude than the desired signal. In general, other considerations will limit its application to some extent. For example, if the circuit preceding the control point is made too broad the continuous random hiss-like interferences caused by thermal agitation and shot efiect may be increased to an undesirable extent. Likewise signals from unwanted powerful radio transmitters on nearby frequencies may be received in sufiicient amplitude to cause overloading in some part of the receiver, resulting in cross modulation and the production of interference which cannot be eliminated by subsequent selectivity. Furthermore the selectivity following the control point must be limited by the requirement of satisfactory transmission of the useful signals to be received.

The voltage (or current) used for actuating the noise reduction control may be obtained from the receiver proper, with or without additional amplification, or from a separate receiver. or other source. If a separate receiver is used, it may or may not use the same antenna as the main re- 5 ceiver. Any one of these arrangements may be advantageous in certain cases. In my application Ser. No. 76,363, filed April 25, 1936, I describe an application of the general rapid acting control method for noise reduction in which the control 10 voltage is derived from the receiver proper, with one additional stage of amplification. In the present disclosure I shall describe an application of the general method in which the control voltage is obtained through a channel entirely sepa- 15 rate from the receiver proper.

When a radio receiver must be located near an electrically ignited internal combustion engine, it is very difficult to prevent the engine ignition system from producing serious interference in the 20 receiver. Such a case .arises, for example, when a radio receiver is to be used in an automobile, airplane, boat, or other vehicle propelled by the usual forms of gas engine. The initial noise pulse produced by the ignition spark, while very intense, is of very short duration, of the order of a few microseconds. However, it shock excites the resonant circuits of the receiver, producing at the output noise pulses of much smaller relative amplitude but much longer in duration, of the order of 250 microseconds, and these may be very disturbing. Therefore such a case is a favorable one for the application of the rapid acting control method for noise reduction. In this case, too, the method may be applied most effectively, as will be clear from what follows. Since the source of interference is localized and very intense, the control voltage may be obtained direct from the ignition system, in practical and usable magnitude, with little or no amplification, 40 through a separate channel'or transmission line. Such a channel may be made very fast in operation, and so the control may be applied and removed very rapidly, thus minimizing the time during which the reception is interrupted and the disturbing effect of the interruption.

Since the control voltage is obtained through a separate channel, the control may be applied at any point in the receiver. The control will be most effective if applied to the first vacuum tube in the receiver, in accordance with the principle of the apportionment of selectivity discussed above. In this case, with the noise reduction control circuit functioning properly, only the input circuit to the grid of the first tube will be shock excited. The resulting noise pulse at the grid of the first tube will be of relatively short duration because the input circuit up to this point will be quite broad, involving usually only one sharply resonant circuit. To accomplish the desired noise reduction, the first tube will need to be cut ofi by the control only for the duration of this relatively brief pulse, and practically the entire selectivity of the receiver will be available to smooth out the disturbance in the receiver output caused by these brief periods during which the control is applied.

In order to protect the receiver beyond the control point from shock excitation, the control must be applied before the interfering impulse reaches the control point. Ordinarily this will be the case if the control channel is made as fast as possible, for other requirements will serve to make the signal channel slower. For example, it will usually be necessary to resonate, with moderate sharpness at least, the grid circuit of the first tube, in order to prevent interference caused by thermal agitation, shot eiiect, or by other radio transmitters from becoming serious. This tuned circuit will delay the arrival and building up of the noise pulse on the grid of the controlled tube, so that it may be cut off by the rapid control circuit before the disturbing noise pulse arrives. If the control cannot be applied in this way in a particular case in sufiicient efiective anticipation of the noise, then one or several of a number of methods may be employed to achieve the desired anticipatory functioning of the control.

In the first place, the desired time difference may be obtained by making the transmission line connecting the antenna to the receiver longer than the transmission line connecting the ignition system to the control tube.

Secondly, the velocity of propagation of the noise impulse along the transmission line connecting the antenna to the receiver may be reduced by including in the vicinity of the line materials having dielectric constants or permeabilities greater than unity. For example iron wire may be used, or a magnetic shield, or insulation of particularly high dielectric constant, or any combination of these expedients.

Thirdly, an artificial line or delay network may be inserted in the connection between antenna and receiver. Any of the well known forms of such circuits may be used, such as low pass and band pass filters.

These methods for slowing up the signal channel may be practically applied in many ways over a wide variety of cases, since this channel will in general be required to transmit efiiciently over only a relatively narrow band of frequencies not greatly wider than that required for the desired signals. For, although to keep the duration of the resultant noise pulse as short as possible and so improve the noise reduction, this signal input circuit should be responsive to as Wide a frequency band as possible, it will usually not be practical to make it very wide because of thermal agitation and shot effect noise and interference from undesired radio transmissions.

On the other hand, it will be practical to make the control channel very rapid in action, because it can be shielded to exclude all interference from radio stations, and because thermal noise or shot eifect will not trouble on account of the high energy level of the actuating noise impulse derived from the ignition system. Thus it can be made to respond to a very wide band of frequencies, and so will transmit the noise impulse with very little distortion or time delay. For example, a high frequency concentric cable such as has been developed for television or the like may be used.

Instead of obtaining the desired efiective anticipation as described above, by a difference in transmission time over the lines actuating the receiver and the control, assuming the noise impulse to occur simultaneously at the input of each, we may excite the input of the control channel by a minor noise impulse or its equivalent preceding by the necessary time interval the excitation of the antenna by the major disturbing noise. This may be accomplished by a proper selection of the point in the ignition system to use for exciting the control channel, and the method for obtaining this excitation. The main noise disturbance is doubtless produced by the main spark which ignites the charge in the engine cylinder, and the resultant voltage (or current) transient in the secondary ignition circuit. This follows with a small time lag the breaking of the primary ignition circuit. The primary circuit is made by the distributor just an instant before the break. Thus the noise reduction control circuit may be energized by the primary ignition circuit, either on the make or break, to give the desired anticipation.

Another method for accomplishing the same result would be to use an auxiliary distributor, advanced in phase relative to the main distributor and driven synchronously with it, especially for the purpose of generating impulses of the desired amplitude, duration and tuning for actuating the noise reduction control circuit.

The invention will be better understood from the following description when considered in connection with the accompanying drawings and its scope will be pointed out in the appended claims.

Referring to the drawings-- Fig. 1 is a wiring diagram of a noise suppressor system including a pair of push-pull connected electron tubes of the. SL7 (RCA Radiotron) type which include a plurality of control grids each capable of independent action on the electron stream.

Fig. 2 is a wiring diagram of a noise suppressor system including a pair of push-pull connected 6J7 (RCA Radiotron) type electron tubes which are susceptible of sharp cut-off high gain operations, the noise suppressing potentials in this modification being applied to the same grids as the signal impulses instead of to separate control grids as in the arrangement of Fig. 1.

Fig. 3 is a wiring diagram of a noise suppressor system including a pair of push-pull connected electron tubes of the 6K7 (RCA Radiotron) type which has a remote cut-off characteristic and is especially suited to handle unusual signal voltages without cross-modulation and modulation distortion thus facilitating its use in connection with automatic volume control.

In this system the desired signal is picked up by antenna 9, together with the extraneous noise impulses which must be suppressed. This antenna is connected by means of shielded lead-in '33 and transformer II] to the input of the first amplifier stage in the receiver, comprising the electron tubes 13 and M in a push-pull connection. A push-pull stage is not essential at this point, but is desirable as will be explained later. The secondary of transformer It] may be tuned. by capacitor l2. Automatic volume control voltage is applied in the usual manner to control grids 2| of electron tubes 13 and M by means of lead 35, and the automatic volume control circuit is by-passed for currents of signal frequency by capacitor H. A fixed bias is applied in the usual manner by means of thevoltage drop across resistor l8 caused by the plate currents of both tubes flowing through it. Resistor I8 is bypassed for currents of signal frequency by means of capacitor I9. The ouput circuit of this stage is comprised of transformer IS, the secondary of which may be tuned'by capacitor l5. D. 0. plate potential is supplied by means of a mid-tap in the primary, and the source of this potential is bypassed for currents of signal frequency by capacitor H. The vacuum tubes I3 and I4 may be of the multi-grid type known in the trade as the RCA Radiotron 6L7 and preferably each includes a grid 2! to which the signal impulses are applied, a screen grid 22, a suppressor grid 23 and a grid 24 to which the noise suppressing impulses are applied.

These noise suppressing impulses are picked up by a member 25, which may be the high tension lead of an automobile or other ignition system and are applied from this member through a capacitor 26, a shielded line 21 and a capacitor 28 to the input circuit of a gaseous discharge control tube 29 which has its cathode grounded to the shield 20 and which may be of type known in the trade as the RCA-885. Connected across the output of the tube 29 are a capacitor 30, a reactor 3! and a resistor 32 from the adjustable terminal of which noise control or suppressing impulses are applied to the grids 24 of the push-pull connected tubes l3l4.

The noise suppressing system of Fig. 2 is similar to that of Fig. 1 in most respects but differs therefrom in that the noise suppression control is applied to the control grids 2| of the two 6J7 sharp cut-off pentode tubes instead of the automatic volume control grids 24 of Fig. 1. This arrangement has the advantage that it involves the use of only a single control grid but involves somewhat greater difiiculty with respect to the simultaneous application of automatic volume control without interaction between the two automatically controlled circuits. The other features of this modification of the invention will be I readily understood without detailed explanation.

The noise suppressor system of Fig. 3 is similar to those of Figs. 1 and 2 but difiers therefrom in that the noise suppression control potential is applied to the cathode (B) connection of two push-pull connected 6K7 remote cut-off tubes. This system possesses the same advantages as the system of Fig. 2 and involves less difficulty in the application of automatic volume control. As compared with the system of Fig. 2, the system of Fig. 3 involves somewhat greater difficulty for the reason that the adjustments of the control and the normal bias of the tubes are so interrelated that flexibility of choice of circuit constants is somewhat restricted.

The RCA Radiotron 6L7 is a convenient tube to use for the control. The No. .1 grid 2| should be used for the signal control grid, since this grid has a remote cut-off characteristic desirable for the first tube in a radio receiver to reduce modulation difliculties with strong interfering signals. The No. 3 grid 24 has a sharp cutoff characteristic and so is desirable for use for the noise reduction control. Unfortunately the lead from this grid is brought out at the base of the tube, and so the effective direct capacitance between this grid and the plate is greater than desirable. It is of the order of 0.25 micromicrofarad, whereas the direct capacity between the plate and grid No. 1, which is brought out at the top of the tube, is only about .005 micromicrofarad. However, there is no apparent reason why this larger capacity could not be made as small as the smaller one by a suitable tube construction. It is essential for noise reduction that both these capacities be small, since if the capacity between plate and signal control grid is large the grid circuit can excite the plate circuit through this capacity even with the tube cut-off. And if the capacity between plate and noise control grid is large, the actuating of the noise control will excite the plate circuit through this capacity.

With the noise reduction control as described above in proper operation, there will still remain a small amount of interference caused by the sudden interruption of the D. C. plate current of the tube controlled, by the operation of the control. This will shock excite the resonant circuits followingthe control point to some extent depending on how rapidly the control is applied. However, this residual disturbance may be eliminated or at least greatly reduced by using a push pull stage for control, connecting the signal control grids and plates in push pull, and the noise control grids in parallel. In this way the D. C. pulses due to the operation of the control will be out of phase for the two tubes, and so will more or less completely be balanced but from the grid circuit of the following tube. This connection will also approximately balance out the residual capacity coupling from noise control grid to plate.

The method shown for obtaining the control voltage pulse of the desired shape and duration and timing involves the use of a gas filled tube, such as an RCA-885 tube, which is triggered by the noise impulse derivedfrom the high tension lead of the ignition system and transmitted over the rapid control transmission line. Of course the gas tube must break down with suiicient rapidity, that is, its ionization time must be small.

This trigger circuit should be designed as follows. The plate and grid voltages of the gas tube must be so chosen that the tube will not ionize without the aid of a positive impulse on its grid which will be obtained from the ignition system. Then the resistor 34 must be chosen so that the condenser 30 will become substantially completely charged between successive noise impulses. The duration of the pulse of control voltage, once started by the ionization of the tube, may be varied by changing the natural period of the circuit 30-3l-32. If this circuit is oscillatory the control voltage pulse will approximate a half cycle of this frequency. The shape of this pulse may be varied by changing the damping by varying the value of the resistor 32. The circuit may be overdamped if this appears desirable. The magnitude of the control voltage pulse may be adjusted to the desired value by means of a slider on the resistor 32.

It is evident from the above that by suitably choosing the various constants of the control actuating circuit shown, practically any desired characteristic can be obtained. However, the duration of the control pulse should be so chosen that the first stage of the receiver is not cut-off any longer than is necessary to prevent shock excitation of the succeeding stages by the noise; and the shape of the control pulse should be so chosen as to minimize the shock excitation of the succeeding circuits by the operation of the control itself, without making the duration of the pulse excessive.

I claim as my invention? 1. A noise reduction system including an amplifier, means including a relatively slow acting circuit for applying to the input of said amplifier desired signals upon which noise impulses may be superposed, and a relatively fast acting circuit including a gas filled tube provided with an input circuit energized by said noise impulses and with an output circuit including an impedance device, and a tuned circuit connected across said output circuit for applying to said amplifier control impulses similar to but displaced in phase with respect to said superposed noise impulses.

2. A noise suppressor system including a pair of input circuits, a pair of electron tubes having control grids connected to said input circuits respectively in push-pull and in parallel, means for applying desired signals to said input circuit to Which said grids are connected in push-pull, and means for applying a series of control impulses of predetermined relatively short duration to said input circuit to which said grids are connected in parallel, and means for determining the shape, duration and timing of said impulses.

3. A noise suppressor system including a pair of input circuits, a pair of electron tubes having control grids connected to said input circuits respectively in push-pull and in parallel, means including a relatively slow acting circuit for applying desired signals to said input circuit to which said grids are connected in push-pull means including a relatively fast acting circuit for applying a series of control impulses of predetermined relatively short duration to said input circuit to which said grids are connected in parallel, and means for determining the shape, duration andtiming of said impulses.

4. A noise suppressor system including a pair of input circuits, a pair of electron tubes having control grids connected to said input circuits respectively in push-pull and in parallel, means for applying desired signals to said input circuit to which said grids are connected in push-pull, means for applying control impulses to said in- 'put circuit to which said grids are connected in parallel, and means for adjusting the duration,

shape and magnitude of said control impulses.

5. The combination of a signal amplifier including a plurality of input circuits, a shielded channel for applying to one of said input circuits a control potential dependent on noise and signal impulses, a gas filled tube provided with a shielded input circuit arranged to be energized in accordance with said noise impulses and with an output circuit including means for determining the shape, duration and timing of the impulses delivered by said tube, and means for applying said delivered impulses to the input circuit of said amplifier.

6. The combination of a signal amplifier including a plurality of input circuits, a channel for applying to one of said input circuits a control potential dependent on noise and signal impulses, and a channel independently associated with the source of said noise impulses and including a gas filled tube provided With a tuned output circuit for applying control potential to another of said input circuits.

7. The combination of a signal amplifier including an input circuit, a channel for applying to said input circuit a control potential dependent on noise and signal impulses, and a fast acting channel independently associated with the source of said noise impulses and including a gas filled tube provided with a tuned output circuit for applying to said input circuit a control potential dependent only on said noise impulses.

8. A radio receiver including a plurality of amplification stages each including an input circuit, a channel for applying to the input circuit of the first of said stages a control potential dependent on noise and signal impulses, and a fast acting channel and including a gas filled tube provided with a tuned output circuit for applying to said input circuit control potential dependent only on said noise impulses.

CHARLES M. BURRILL. 

